Nature: “Dynamic thinning of Greenland and Antarctic ice-sheet ocean margins is more sensitive, pervasive, enduring and important than previously realized”

The most detailed satellite information available shows that ice sheets in Greenland and western Antarctica are shrinking faster than scientists thought and in some places are already in runaway melt mode, a new study found….

Using 50 million laser readings from a NASA satellite, scientists for the first time calculated changes in the height of the vulnerable but massive ice sheets and found them especially worse at their edges. That’s where warmer water eats away from below. In some parts of Antarctica, ice sheets have been losing 30 feet a year in thickness since 2003, according to the study….

“To some extent it’s a runaway effect. The question is how far will it run?” said lead author Hamish Pritchard of the British Antarctic Survey. “It’s more widespread than we previously thought.”

The British Antarctic Survey put out a news release with graphics. Here are some satellite tracks, from NASA’s ICESat (Ice, Cloud, and land Elevation Satellite), revealing areas of dynamic thinning (red) in Antarctica and Greenland [click to enlarge].

The release notes that this “dynamic thinning”:

reaches all latitudes in Greenland

has intensified on key Antarctic coastlines

is penetrating far into the ice sheets’ interior and

is spreading as ice shelves thin by ocean-driven melt.

The authors conclude “Ice shelf collapse has triggered particularly strong thinning that has endured for decades.” More of the MSNBC story:

Some of those areas are about a mile thick, so they’ve still got plenty of ice to burn through. But the drop in thickness is speeding up. In parts of Antarctica, the yearly rate of thinning from 2003 to 2007 is 50 percent higher than it was from 1995 to 2003.

These new measurements confirm what some of the more pessimistic scientists thought: The melting along the crucial edges of the two major ice sheets is accelerating and is in a self-feeding loop. The more the ice melts, the more water surrounds and eats away at the remaining ice.

Antarctica is disintegrating much faster than almost anybody imagined. In 2001, the IPCC “consensus” said neither Greenland nor Antarctica would lose significant mass by 2100. They both already are. As Penn State climatologist Richard Alley said in March 2006, the ice sheets appear to be shrinking “100 years ahead of schedule.”

The warming of the WAIS is most worrisome (at least for this century) because it’s going to disintegrate long before the East Antarctic Ice Sheet does “” since WAIS appears to be melting from underneath (i.e. the water is warming, too), and since, as I wrote in the “high water” part of my book, the WAIS is inherently less stable:

Perhaps the most important, and worrisome, fact about the WAIS is that it is fundamentally far less stable than the Greenland ice sheet because most of it is grounded far below sea level. The WAIS rests on bedrock as deep as two kilometers underwater. One 2004 NASA-led study found that most of the glaciers they were studying “flow into floating ice shelves over bedrock up to hundreds of meters deeper than previous estimates, providing exit routes for ice from further inland if ice-sheet collapse is under way.” A 2002 study in Science examined the underwater grounding lines-the points where the ice starts floating. Using satellites, the researchers determined that “bottom melt rates experienced by large outlet glaciers near their grounding lines are far higher than generally assumed.” And that melt rate is positively correlated with ocean temperature.

The warmer it gets, the more unstable WAIS outlet glaciers will become. Since so much of the ice sheet is grounded underwater, rising sea levels may have the effect of lifting the sheets, allowing more-and increasingly warmer-water underneath it, leading to further bottom melting, more ice shelf disintegration, accelerated glacial flow, and further sea level rise, and so on and on, another vicious cycle. The combination of global warming and accelerating sea level rise from Greenland could be the trigger for catastrophic collapse in the WAIS(see, for instance, here).

You can read every thing a laymen could possibly want to know about what the recent study on Antarctic warming does and doesn’t show at RealClimate here.

The authors of the Nature article find:

In Antarctica, we find significant dynamic thinning of fast-flowing ice at rates greater than plausible through interannual accumulation variability for drainage sectors…. On the glacier scale, thinning is strongest in the Amundsen Sea embayment (ASE), where it is confirmed as being localized on the fast-flowing glaciers and their tributaries (Fig. 3 [below]. The area close to the Pine Island Glacier grounding line thinned in the period 2003–2007 at up to 6 m yr-1, neighbouring Smith Glacier thinned at a rate in excess of 9 m yr-1 and Thwaites Glacier thinned at a rate of around 4 m yr-1. These rates are higher than those reported for the 2002–2004 period.

They conclude:

In Antarctica, dynamic thinning has accelerated at the grounding lines of the major glaciers of the Amundsen Sea embayment, and in places has penetrated to within 100 km of the ice divides. Ice-shelf-collapse glaciers show particularly strong thinning that has persisted for years to decades after collapse and in places has penetrated to their headwalls. Although losses are partly offset by strong gains on the spine and western flank of the Antarctic Peninsula, numerous glaciers feeding intact Antarctic Peninsula, West Antarctic and East Antarctic ice shelves are also thinning dynamically. We infer that grounded glaciers and ice streams are responding sensitively not only to ice-shelf collapse but to shelf thinning owing to ocean-driven melting. This is an apparently widespread phenomenon that does not require climate warming sufficient to initiate ice-shelf surface melt. Dynamic thinning of Greenland and Antarctic ice-sheet ocean margins is more sensitive, pervasive, enduring and important than previously realized.